Speed Card-Controlled Override Fuel Pump Assist

ABSTRACT

A system and method for supplementing fuel feed pressure and flow within an aircraft fuel system. The fuel system includes boost and override fuel pumps delivering fuel from the tanks to a fuel manifold, and a jettison fuel pump. The method includes the steps of: (a) sensing whether the aircraft engine is operating near maximum power; (b) upon sensing the condition, operating the jettison fuel pump in fluid interconnection with the override fuel pump to deliver fuel to the fuel manifold; and (c) upon sensing the cessation of the condition, deactivating the jettison fuel pump. The system includes a monitoring circuit signaling when the aircraft engine speed is greater than a predetermined threshold, and a fuel system control circuit operating a jettison fuel pump enable circuit portion in response to the signal while omitting other jettisoning operations. The jettison fuel pump consequently functions as an override fuel pump assist.

FIELD

The present disclosure is directed to systems and methods for thedistribution of fuel within a jet aircraft and, in particular, tosystems and methods for supplementing fuel feed pressure and flow withina jet aircraft having multiple fuel tanks.

BACKGROUND

Large, multiple engine jet aircraft typically have a fuel system whichincludes multiple fuel tanks in order to provide long-haul capabilitywhile maintaining a proper distribution of weight. In the case of aBoeing 767-300, the aircraft has a primary fuel tank or “wing tank”holding up to 18,500 kgs of fuel in each of the aircraft's wings, and asingle auxiliary fuel tank or “center wing tank” holding up to 36,500kgs of fuel in the aircraft's center wing box. The fuel tanks arefluidly connected to the aircraft's twin engines by a fuel manifold,with the wing tanks and the center wing tank being fluidly connected tothe fuel manifold through a cross-feed manifold portion running throughthe center wing box. This cross-feed manifold portion is divided intotwo halves by at least one cross-feed valve, with the center wing tankbeing fluidly connected to both halves of the cross-feed manifoldportion in order to create two operationally separate, yet selectivelyinterconnectable, multiple tank subsystems for distributing fuel to theengines.

Fuel is pumped from the primary and auxiliary fuel tanks into the fuelmanifold by a plurality of fuel pumps. In the case of a Boeing 767-300,each wing tank includes two low pressure boost fuel pumps, both of whichare fluidly connected the same half of the cross-feed manifold portionfor sake of redundancy, while the center wing tank includes two highpressure fuel pumps, each of which is fluidly connected to a differenthalf of the cross-feed manifold portion in order to permit independentauxiliary fueling of the aircraft engines. In most aircraft fuelsystems, and under ordinary conditions, the auxiliary tank is operatedin an override mode, meaning that the low pressure boost fuel pumps in aprimary tank remain active regardless of whether fuel is to be drawnfrom the primary tank or the auxiliary tank. When fuel is to be drawnfrom the auxiliary tank, a high pressure fuel pump or “override fuelpump” is activated to deliver fuel to the fuel manifold at a greaterpressure than the low pressure boost fuel pumps can generate, overridingthe flow of fuel from the boost fuel pumps into the fuel manifold and,in effect, de-selecting the primary tank. On the other hand, when fuelis to be drawn from the primary tank, then the override fuel pump isdeactivated, and the flow of fuel from the boost fuel pumps into thefuel manifold resumes. In short, in an override-based aircraft fuelsystem, the system will include both low pressure boost fuel pumps andhigh pressure override fuel pumps, with the boost fuel pumps remainingactive in order to prevent an interruption in fuel flow which couldresult in an engine flameout, and fuel manifold pressure controllingwhether fuel will be drawn from a particular fuel tank.

In some aircraft, factors such as the specific performance capabilitiesof the installed boost and override fuel pumps, the backpressure profileof the fuel manifold, and the suction pressure generated by an aircraftengine's own fuel pumps can diminish the fuel pressure proximate theboost fuel pumps and cause an “incomplete override” of one or more ofthe pumps, allowing fuel to be unintentionally drawn from a primary fueltank, e.g., a wing tank, rather than an auxiliary fuel tank, e.g., acenter wing tank. Such a situation may occur, for example, if an engineis operating near maximum power during the aircraft's initial climb andone of the installed boost fuel pumps is capable of pumping atgreater-than-average pressures due to pump-to-pump manufacturingvariations. Because the amount of fuel drawn from various fuel tankswill vary in an incomplete override scenario depending upon theindividual performance capabilities of the fuel pumps, fuel may be drawnunevenly from the aircraft's fuel tanks, adversely affecting theaircraft's distribution of weight, in addition to contravening flightpractices which require that auxiliary fuel supplies be essentiallyexhausted before switching over to primary fuel supplies. This unplannedand, likely, uneven fuel draw may require frequent adjustments toaircraft trim and repeated rebalancing of the aircraft's fuel load,increasing both pilot workload and opportunities for the risksassociated with in-flight fuel transfers to become manifest.

The applicant has developed a system and method for supplementing fuelfeed pressure and flow without requiring the replacement of existingfuel pumps in existing aircraft, the replacement of existing fuel pumpdesigns in existing aircraft designs, and the like. The disclosed systemand method enable the automatic control of a multi-tank, multi-pump,override-based aircraft fuel system using pre-existing fuel pumps orfuel pump designs, in combination with novel modifications to theaircraft's engine monitoring circuity and fuel system control circuitry,in order to assist the override fuel pump. The system and method thusmitigate a risk of incomplete override with substantially reducedengineering cost and, potentially, substantially reduced maintenancecosts and out-of-service delays.

SUMMARY

In one form, a method for supplementing fuel feed pressure and flowwithin an aircraft fuel system fueling an aircraft engine. The aircraftfuel system includes a primary fuel tank, a first boost fuel pumpdelivering fuel from the primary tank to a fuel manifold, an auxiliaryfuel tank, an override fuel pump delivering fuel from the auxiliary tankto the fuel manifold, and a jettison fuel pump operable to draw fuelfrom the auxiliary fuel tank. The method includes the steps of: (a)sensing whether the aircraft engine is operating near maximum power; (b)upon sensing that the aircraft engine is operating near maximum power,operating the jettison fuel pump in fluid interconnection with theoverride fuel pump to deliver fuel to the fuel manifold; and (c) uponsensing that the aircraft engine has ceased to operate near maximumpower, deactivating the jettison fuel pump. The jettison fuel pump maybe a component of a fully-installed jettison system, with the operatingstep omitting any jettisoning operation of a jettisoning valve, or maybe a component of a partially-installed jettison system, with thejettison fuel pump functioning only as an override fuel pump assist.

In another form, an override-based aircraft fuel system including a fuelmanifold, a primary fuel tank having a first boost fuel pump fluidlyconnected to the fuel manifold, and an auxiliary fuel tank having bothan override fuel pump, fluidly connected to the fuel manifold, and ajettison fuel pump, fluidly connectable to the fuel manifold. The fuelsystem further includes an engine monitoring circuit including (a) anaircraft engine speed sensor operatively connected to a comparator and(b) a control relay selectively providing a signal output, with thecomparator and control relay providing the signal output when theaircraft engine speed sensor detects an aircraft engine speed greaterthan a predetermined threshold. The signal output is communicated to afuel system control circuit including a first operating relayoperatively connected to a jettison fuel pump enable circuit portion,where the signal output operates the first operating relay and thejettison fuel pump enable circuit portion to operatively control thejettison fuel pump and deliver fuel to the fuel manifold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a first partial schematic diagram of an exemplary aircraftfuel system, joining to FIG. 1B along the former figure's left edge.

FIG. 1B is a second partial schematic diagram of an exemplary aircraftfuel system, joining to FIG. 1A along the former figure's right edge.

FIG. 2 is a block diagram of an aspect of the disclosed method.

FIG. 3A is a partial schematic diagram of an exemplary aircraft enginemonitoring circuit, also commonly known as an “engine speed card.”

FIG. 3B is a partial schematic diagram of an exemplary aircraft fuelsystem control circuit including jettison system valve controlcircuitry.

DETAILED DESCRIPTION

With initial reference to FIG. 1A, a first aspect of the disclosedsystem and method comprises an improvement to an existing aircraft fuelsystem 100. The aircraft fuel system 100 includes a first main fuel tankor primary tank 110, which may be disposed within a wing as a so-called“wing tank,” and a second main fuel tank or auxiliary tank 120, whichmay be at least partially disposed within the aircraft's centralfuselage as a so-called “center wing tank” or the like. The primary tank110 includes at least a first boost fuel pump 112, and in typicalvariations may include a second boost fuel pump 114. The auxiliary tank120 includes an override fuel pump 122 and, in a first embodiment, alsoincludes a jettison fuel pump 124. A fuel manifold 130 interconnects theprimary and auxiliary tanks 110, 120 with an aircraft engine (notshown), with the first and second boost fuel pumps 112, 114 beingfluidly connected to the fuel manifold 130 to deliver primary fuel tothe aircraft engine, and the override fuel pump 122 being fluidlyconnected through a check valve to the fuel manifold 130 to deliverauxiliary fuel to the aircraft engine. In multiple engine aircraft, theboost fuel pump 112 (and 114, if present) and the override fuel pump 122may be connected to a cross-feed manifold portion 140, with thecross-feed manifold portion 140 including at least one cross-feed valve142 to divide the fuel system 100 into interconnectable subsystems, suchas left and right subsystems. As indicated in the brief description ofthe drawings, the right subsystem of the fuel system 100 shown in FIG. 1has been omitted, but it will be understood that the exemplary systembeing discussed herein is essentially bilaterally symmetric, withduplicate components and connections being provided for the two halvesof the exemplary system.

Jettison fuel pump 124 may be a component of a pre-existing,fully-installed jettison system 150, shown in FIGS. 1A and 1B, whichtypically includes a jettison transfer valve 152 fluidly connected tothe jettison fuel pump 124, a jettison nozzle valve 154 fluidlyconnected to the jettison transfer valve 152, and a jettison nozzle 156fluidly connected to the jettison nozzle valve 154. The jettison nozzle156 is typically disposed near the trailing edge of the aircraft wing,with the jettison system 150 being manually activated in order to bringthe aircraft below its maximum safe landing weight in the event that aflight must land soon after a heavily-laden takeoff. As shown FIG. 1A,it is known to configure both the override fuel pump 122 and thejettison fuel pump 124 to deliver fuel to a common manifold for releasethrough the jettison system 150, so that the jettison fuel pump 124 andthe override fuel pump 122 may be used to jettison fuel at roughly twicethe rate at which fuel would be jettisoned by the jettison fuel pump 124alone. However, the applicant has determined that the combination of thejettison fuel pump 124 and the override fuel pump 122, with appropriatemodifications to the aircraft's engine monitoring and fuel systemcontrol circuity, is also capable of delivering fuel through the checkvalve connection to the fuel manifold 130 at an increased ratesufficient to compensate for manufacturing variability in thecapabilities of the boost fuel pump 112 (and 114, if present), thebackpressure of the fuel manifold 130, the suction forces generated bythe aircraft engine pumps, etc., and improve upon the pump-head capacitycurve of the override fuel pump 122. In such an alternate mode, jettisonfuel pump 124 functions as an override fuel pump assist, lowering therisk of incomplete override by decreasing pressure losses at high fuelflow rates and, in particular, increasing fuel pressure proximate thebost fuel pumps.

Referring now to FIG. 2, in a method of supplementing fuel feed pressureand flow relating to the first aspect, the aircraft fuel system 100 isoperated according to the following steps:

First, 210, sensing whether the aircraft engine is operating nearmaximum power. For the purposes of this application, the phrase “nearmaximum power” should be interpreted to mean 70% percent or more ofmaximum rated power. In a first variation of step 210, the sensing stepsenses whether the aircraft engine speed is above a predeterminedthreshold. In first example of this first variation, the predeterminedthreshold is a rotor speed of an N2 or “intermediate pressurecompressor” rotor of the aircraft engine. In a first specific example ofthis first variation, the predetermined threshold is about 83% of themaximum rated rotor speed of the N2 rotor of the aircraft engine. In asecond example of this first variation, the predetermined threshold mayinclude a hysteresis filter having a first rotor speed of an N2 rotorfor generally increasing rotor speeds and a second rotor speed of the N2rotor for generally decreasing rotor speeds, where the first rotor speedis greater than the second rotor speed. In a second specific example ofthis first variation, the first rotor speed is about 83% of the maximumrated rotor speed of the N2 rotor of the aircraft engine and the secondrotor speed is about 72% of the maximum rated rotor speed of the N2rotor of the aircraft engine. In a second variation of step 210, thesensing step senses whether the aircraft engine throttle setting isabove a predetermined threshold. A similar simple value threshold or asimilar threshold including a hysteresis filter may be implemented asexamples of this second variation. A similar threshold of about 83% ofmaximum throttle of the aircraft engine or a similar hysteresis filterof 83% (increasing)/72% (decreasing) of maximum throttle of the aircraftengine may be implemented as specific examples of this second variation.

The sensing step 210 may be automated using an engine monitoring circuit300 such as an “engine speed card.” For example, FIG. 3A shows aschematic diagram of an exemplary engine monitoring circuit 300 from aBoeing 767 series aircraft. An engine speed sensor 310 detects the rotorspeed of an N2 rotor in the aircraft engine. The engine speed sensor 310is operatively connected to a comparator 320 having a hysteresis filterwhich triggers at 83% of the maximum rated rotor speed of the N2 rotorof the aircraft engine for generally increasing speeds, and untriggersat 72% of the maximum rated rotor speed of the N2 rotor of the aircraftengine for generally decreasing speeds. The comparator drives a controlrelay 330, which provides an on/off signal output 340 to a fuel systemcontrol circuit 400 as further described below. In one particularimplementation, the control relay 330 obtains signal power from thejettison system control circuitry and, more specifically, from a powersupply lead within the jettison system control panel 402 (as shown inFIG. 3B). Those of skill in the art will of course recognize that signalpower may be obtained from a variety of sources within the aircraft.

Returning to FIG. 2, second, 220, upon sensing that the aircraft engineis operating near maximum power, operating a jettison fuel pump 124 influid interconnection with the override fuel pump 122 to fuel theaircraft engine. In the aforementioned first variation, the condition isof course based upon whether the aircraft engine speed is above thepredetermined threshold, while in the aforementioned second variation,the condition is based upon whether the aircraft engine throttle settingis above the predetermined threshold. In general, the operating step 230omits any jettisoning operation of the jettison transfer valve 152 andthe jettison nozzle valve 154 so that operation of the jettison fuelpump 124 delivers fuel to the fuel manifold 130. However, in variationsof the second step as applied to other aircraft, the operating step mayrequire the opening of one or more other transfer valves to place thejettison fuel pump 124 in fluid communication with the override fuelpump 122, either directly or indirectly through the fuel manifold 130,e.g., through the cross-feed manifold portion 140, in order to configurethe fuel system for override assist.

The operating step 220 may be automated using a fuel system controlcircuit 400 which otherwise operates the jettison system 150. Forexample, FIG. 3B shows a schematic diagram of an exemplary jettison fuelpump control circuit 400 from a Boeing 767 series aircraft. The signaloutput 340 from control relay 330 (shown FIG. 3A) operates a firstoperating relay 410 operatively connected to a jettison fuel pump enablecircuit portion 420, which operatively controls the jettison fuel pump124 (shown as a functional block for reference purposes). In oneparticular implementation, the first operating relay 410 is operativelyconnected to an input of an electrical load shedding portion 422 of thejettison fuel pump enable circuit portion 420 and to an input of a GFIportion 424 of the jettison fuel pump enable circuit portion 420, withthe latter operatively controlling the jettison fuel pump 124. The firstoperating relay 410 passes the signal output 340 to the inputs of theportions 422 and 424 when operated by the signal output 340. Those ofskill in the art will of course recognize that the first operating relay410 and the jettison fuel pump enable circuit portion 420 may beconfigured in various manners, so long as the combined first operatingrelay 410 and jettison fuel pump enable circuit portion 420 operativelycontrols the jettison fuel pump 124. The signal output 340 from controlrelay 330 may also operate a second operating relay 430 operativelyconnected to the jettison fuel pump enable circuit portion 420. In theaforementioned particular implementation, the second operating relay 430is operatively connected to the input of the GFI interrupter portion 424of the jettison fuel pump enable circuit portion 420, providing a pathto ground for the signal output 340 passed to this input, but only whenthe second operating relay 430 is operated by the signal output 340.

Returning to FIG. 2, third, 230, upon sensing that the aircraft enginehas ceased operating near maximum power, deactivating the jettison fuelpump 124. Again, in the aforementioned first variation, the condition isbased upon whether the aircraft engine speed drops below thepredetermined threshold, while in the aforementioned second variation,the condition is based upon whether the aircraft engine throttle settingdrops below the predetermined threshold. In a corresponding firstspecific example of the first variation, where the predeterminedthreshold is about 83% of the maximum rated rotor speed of the N2 rotorof the aircraft engine, the deactivation step occurs after the enginespeed drops below the 83% threshold. In a modification of acorresponding first variation and/or the corresponding first specificexample, the deactivation step may be delayed by a predetermined periodof time after sensing that the aircraft engine speed has dropped belowthe first predetermined threshold. Such a predetermined period of timemay be used in place of a hysteresis filter to prevent transitorychanges in aircraft engine speed from causing the deactivation of thejettison fuel pump 124.

In a corresponding second example of the first variation, where thefirst predetermined threshold includes a hysteresis filter having afirst rotor speed of an N2 rotor for generally increasing rotor speedsand a second rotor speed of the N2 rotor for generally decreasing rotorspeeds, the deactivating step occurs after the aircraft engine speeddrops below the second rotor speed of the hysteresis filter. In acorresponding second specific example of the first variation, where thefirst rotor speed is about 83% of the maximum rated rotor speed of theN2 rotor of the aircraft engine and the second rotor speed is about 72%of the maximum rated rotor speed of the N2 rotor of the aircraft engine,the deactivating step occurs after the aircraft engine speed has droppedbelow the 72% level of the hysteresis filter. In corresponding examplesand specific examples of the second variation, the deactivating stepoccurs after the aircraft engine throttle setting drops below thepredetermined threshold (in the corresponding specific example, belowabout 83% of maximum throttle of the aircraft engine) or the lower levelof the hysteresis filter of the predetermined threshold (in thecorresponding specific example, below the about 72% level of thehysteresis filter), as the case may be. In a modification of the secondvariation, the deactivation step may again be delayed by a predeterminedperiod of time after sensing that the aircraft engine throttle settinghas dropped below the first predetermined threshold in place of the useof a hysteresis filter.

Again, in general, the deactivating step 230 omits any operation of thejettison transfer valve 152 and the jettison nozzle valve 154; however,in variations of the third step as applied to other aircraft, thedeactivating step may require the closing of one or more transfer valvesto remove the jettison fuel pump 124 from fluid interconnection with theoverride fuel pump 122 and/or the fuel manifold 130 in order to returnto a standby configuration or to reconfigure the jettison system 150 forpotential fuel jettisoning.

The deactivating step 230 may be automated using the engine monitoringcircuit 300 and fuel system control circuit 400 previously described inthe context of the sensing step 210 and the operating step 220. Theengine speed sensor 310 is operatively connected to the comparator 320and, in the exemplary circuit illustrated in FIG. 3A, a hysteresisfilter which untriggers at about 72% of the maximum rated rotor speed ofthe N2 rotor of the aircraft engine for generally decreasing speeds. Thecomparator drives the control relay 330 to switch off the signal output340 to the fuel system control circuit 400. This deactivates the firstoperating relay 410 and jettison fuel pump enable circuit portion 420.In the particular implementation previously described, the firstoperating relay 410 deactivates both the electrical load sheddingportion 422 of the jettison fuel pump enable circuit portion 420 and theGFI interrupter portion 424 of the jettison fuel pump enable circuitportion 420, with the latter causing the deactivation of the jettisonfuel pump 124. In that particular implementation, the switching off ofthe signal output 340 also deactivates the second operating relay 430,which disconnects the GFI interrupter portion 424 of the jettison fuelpump enable circuit portion 420 from ground.

In a second aspect of the disclosed system and method, the aircraft fuelsystem 100 may lack a pre-existing jettison system 150. Suchconfigurations may exist where jettison systems are a customer option,and include many twin-engine “regional jets” capable of meeting FAALanding Climb and Approach Climb regulations in lower capacityconfigurations. The second main tank 120 will include a override fuelpump 122 and, in a second embodiment, design provision for an optionaljettison fuel pump 124. The aircraft fuel system control circuit mayalso, in the second embodiment, include pre-existing jettison systemvalve control circuitry 400 or design provision for optional jettisonsystem valve control circuitry 400 through a daughterboard, externalsignal and power bus, or the like. Alternately, the aircraft fuel systemcontrol circuit may be upgradable to an optional aircraft fuel systemcontrol circuit including a partially or fully populated jettisoncontrol circuit.

The aircraft fuel system 100 may subsequently be retrofitted topartially install a jettison system 150 including at least a jettisonfuel pump 124 in fluid communication with the override fuel pump 122. Inaircraft such as the Boeing 767-300, where the override fuel pump 122and jettison fuel pump 124 are designed to discharge into a commonmanifold, the jettison transfer valve 152, jettison nozzle valve 154,and jettison nozzle 156 may be omitted, along with other portions of thejettison system which are unique to that system such as jettisoningextensions to the aircraft refueling manifold 160 (which serves todistribute fuel to the main tanks during ground refueling operations aswell as to deliver fuel to the jettison nozzle valve 154 and jettisonnozzle 156 during jettisoning operations). Thus, the optional jettisonfuel pump 124 would be fluidly connected to the fuel manifold 130 butnot fluidly connected to the aircraft refueling manifold 160, servingonly as an override fuel pump assist. Those of skill in the art will ofcourse recognize that the equipment which can be omitted from a retrofitinstallation of the override fuel pump assist will vary depending uponthe design of the jettison system 150 in each particular type ofaircraft, and that additional manifold structure such as a fuel manifoldextension and check valve may be required to connect the optionaljettison fuel pump 124 to the fuel manifold 130 depending upon theconfiguration of the optional jettison system in that type of aircraft.

The fuel system control circuit 400 is also subsequently be retrofittedto add at least the first operating relay 410 as described above. In avariation, the retrofitted fuel system control circuit 400 also includesthe jettison fuel pump enable circuit portion 420 and second operatingrelay 430 described above. This may require a modification ofpre-existing jettison system valve control circuitry or a modificationof a pre-designed jettison system valve control circuit provided asdaughterboard or bus-connected accessory board, etc. Alternately, thefuel system control circuit 400 may be upgraded to a circuit includingan at least partially populated jettison control circuit that includesat least the jettison fuel pump enable circuit portion 420 and the firstoperating relay 410.

The retrofitted aircraft may subsequently be operated in accordance withthe method described in the context of the first aspect. Automation ofthe method is substantially the same as that described in the context ofthe first aspect, although particulars such as the omission of anyoperation of a jettison transfer valve and jettison nozzle valve may besuperfluous due to the potential absence of these structures in aretrofitted installation.

The various aspects, embodiments, implementations, and variationsdescribed above are intended to be illustrative in nature, and are notintended to limit the scope of the invention. In particular, thedisclosed specific examples and figures, illustrating the systems usedin a twin engine Boeing 767 aircraft, are intended to demonstrate abroader applicability to single or multiple engine jet aircraft ingeneral. Any limitations to the invention will appear in the claims asallowed in view of the terms explicitly defined herein.

1. A method for supplementing fuel feed pressure and flow within anaircraft fuel system fueling an aircraft engine, the aircraft fuelsystem including a primary fuel tank, a first boost fuel pump deliveringfuel from the primary tank to a fuel manifold, an auxiliary fuel tank,an override fuel pump delivering fuel from the auxiliary fuel tank tothe fuel manifold, and a jettison fuel pump operable to draw fuel fromthe auxiliary fuel tank, and the method comprising the steps of: (a)sensing whether the aircraft engine is operating near maximum power; (b)upon sensing that the aircraft engine is operating near maximum power,operating the jettison fuel pump in fluid interconnection with theoverride fuel pump to deliver fuel to the fuel manifold; and (c) uponsensing that the aircraft engine has ceased to operate near maximumpower, deactivating the jettison fuel pump; whereby operation of thejettison fuel pump in combination with the override fuel pump completelyoverrides fuel delivery by the first boost fuel pump from the primaryfuel tank to the fuel manifold.
 2. The method of claim 1, wherein thejettison fuel pump is a component of a fully-installed jettison systemincluding a jettison transfer valve and a jettison nozzle valve, and theoperating step omits any jettisoning operation of the jettison transfervalve and the jettison nozzle valve.
 3. The method of claim 1, whereinthe jettison fuel pump is a component of a partially-installed jettisonsystem, and the jettison fuel pump functions only as an override fuelpump assist.
 4. The method of claim 1, wherein the sensing step senseswhether the aircraft engine speed is above a first predeterminedthreshold.
 5. The method of claim 4, wherein the predetermined thresholdis a rotor speed of an N2 rotor of the aircraft engine.
 6. The method ofclaim 5, wherein the predetermined threshold includes a hysteresisfilter having a first rotor speed of an N2 rotor for generallyincreasing rotor speeds and a second rotor speed of the N2 rotor forgenerally decreasing rotor speeds, where the first rotor speed isgreater than the second rotor speed.
 7. The method of claim 4, whereinthe deactivation step is delayed by a predetermined period of time uponsensing that the aircraft engine speed has dropped below the firstpredetermined threshold.
 8. The method of claim 1, wherein the sensingstep senses whether the aircraft engine throttle setting is above afirst predetermined threshold.
 9. The method of claim 8, wherein thepredetermined threshold includes a hysteresis filter having a firstaircraft engine throttle setting for generally increasing throttlesettings and a second aircraft engine throttle setting for generallydecreasing throttle settings, where the first aircraft engine throttlesetting is greater than the second aircraft engine throttle setting. 10.The method of claim 8, wherein the deactivation step is delayed by apredetermined period of time upon sensing that the aircraft enginethrottle setting has dropped below the first predetermined threshold.11. An override-based aircraft fuel system for fueling an aircraftengine, the aircraft fuel system comprising: a fuel manifold; a primaryfuel tank including a first boost fuel pump fluidly connected to thefuel manifold; an auxiliary fuel tank including an override fuel pumpfluidly connected to the fuel manifold and a jettison fuel pump fluidlyconnectable to the fuel manifold; an engine monitoring circuit including(a) an aircraft engine speed sensor operatively connected to acomparator and (b) a control relay selectively providing a signaloutput, wherein the comparator is configured to drive the control relayto provide the signal output when the aircraft engine speed sensordetects an aircraft engine speed greater than a predetermined threshold;and a fuel system control circuit including a first operating relayoperatively connected to a jettison fuel pump enable circuit portion,wherein the signal output operates the first operating relay and thejettison fuel pump enable circuit portion, and wherein the jettison fuelpump enable circuit portion operatively controls the jettison fuel pumpto deliver fuel to the fuel manifold.
 12. The override-based aircraftfuel system of claim 11, further comprising a jettison transfer valvefluidly connected the jettison fuel pump, wherein the fuel systemcontrol circuit is configured to operate the jettison fuel pump enablecircuit portion and jettison fuel pump without operating the jettisontransfer valve so as to permit the jettisoning of fuel.
 13. Theoverride-based aircraft fuel system of claim 11, wherein the enginespeed sensor senses a rotor speed of an N2 rotor of the aircraft engine.14. The override-based aircraft fuel system of claim 13, wherein thepredetermined threshold of the comparator includes a hysteresis filterhaving a first rotor speed of an N2 rotor for generally increasing rotorspeeds and a second rotor speed of the N2 rotor for generally decreasingrotor speeds, where the first rotor speed is greater than the secondrotor speed.
 15. The override-based aircraft fuel system of claim 14,wherein the first rotor speed is about 83% of the maximum rated rotorspeed of the N2 rotor of the aircraft engine and the second rotor speedis about 72% of the maximum rated rotor speed of the N2 rotor of theaircraft engine.
 16. The override-based aircraft fuel system of claim11, wherein the fuel system control circuit includes a second operatingrelay operatively connected to the jettison fuel pump enable circuitportion, and wherein the signal output operates the second operatingrelay to provide a path to ground for the signal output operating thejettison fuel pump enable circuit portion